(1) Field of the Invention
The present invention generally relates to a malfunction detection apparatus, and more particularly to an apparatus for detecting a malfunction in an evaporated fuel purge system provided in an internal combustion engine, in which fuel vapor evaporated in a fuel tank is adsorbed in an adsorbent in a canister and the adsorbed fuel vapor is purged into an intake passage in given operating conditions.
(2) Description of the Related Art
Conventionally, in an internal combustion engine, an evaporated fuel purge system is provided for storing fuel vapor, which is evaporated in a fuel tank, temporarily in an adsorbent in a canister, and for purging the stored fuel vapor in the adsorbent into an intake passage of the engine under given operating conditions. In order to prevent the evaporated fuel vapor in the fuel tank from escaping to the atmosphere, the component parts, the connecting pipes and the passages in the evaporated fuel purge system are all sealed. However, in a case in which a connecting pipe in the system is separated, or a purge supply passage is damaged due to a trouble in the evaporated fuel purge system, the fuel vapor from the fuel tank may escape to the atmosphere. In a case where the purge passage leading to the intake passage of the engine is clogged with foreign matter, too much fuel vapor is stored in the canister and the excessive fuel vapor may leak from an air inlet of the canister to the atmosphere. Therefore, it is necessary to detect a malfunction which may take place in the evaporated fuel purge system.
In the meantime, in an internal combustion engine equipped with an electronic control type fuel injection control unit, an air-fuel ratio (A/F) feedback control device is provided for maintaining the air-fuel mixture, supplied to the combustion chamber of the internal combustion engine, at a predetermined target air-fuel ratio. For example, Japanese Laid-Open Patent Application No. 63-186954 discloses such an A/F feedback control device, and a description of A/F feedback correction factor included therein is hereby incorporated into the present specification. In this A/F feedback control device, a basic fuel injection time during which fuel is injected to the combustion chamber is calculated on the basis of an intake passage vacuum pressure (or a manifold absolute pressure) and an engine speed, and the calculated basic fuel injection time is adjusted suitably in response to an output signal of an oxygen sensor mounted in an exhaust passage of the engine. Conventionally, the A/F feedback control system uses an air-fuel ratio (A/F) feedback correction factor FAF for correcting the basic fuel injection time, the basic fuel injection time being multiplied by the factor FAF, which is determined in response to the output signal of the oxygen sensor, and several other factors in order to obtain a suitably adjusted fuel injection time. Japanese Laid-Open Patent Application No. 2-136558 discloses a conventional malfunction detecting device for use in the internal combustion engine with the A/F feedback control system. In this malfunction detecting device, a purge control valve is switched on and off in response to the internal pressure of the fuel tank which is higher than a predetermined level, and the change in the A/F feedback correction factor FAF between the times when the purge control valve is switched on and off is detected. And, if the change in the FAF is not greater than a predetermined value, then it is determined by the malfunction detecting device that there is a malfunction in the evaporated fuel purge system. The reasons why the malfunction discrimination is made when the internal pressure is higher than a predetermined level is to prevent the malfunction detection from being erroneously performed due to excessively small amount of fuel vapor which is adsorbed in the canister.
However, in a case of the above described conventional malfunction detecting device, the amount of the adsorbed fuel vapor in the canister is relatively small immediately after the internal pressure of the fuel tank reaches the predetermined level. The change in the FAF in such cases is too small, and it is difficult to accurately detect a malfunction in the evaporated fuel purge system, and an erroneous malfunction detection may be made in some cases.
One conceivable method for storing the amount of fuel vapor in the canister, required for accurate malfunction detection, is to stop the purging of fuel vapor into the intake passage for a relatively long time period. However, during the purging stop time period, the temperature of fuel in the fuel tank becomes very high. Also, in a case in which a fuel of the kind including much evaporative components is supplied, unnecessarily abundant fuel vapor is adsorbed in the canister. For these reasons, the above mentioned purge stopping method also has a problem in that the adsorbing capacity of the canister is lowered from its normal level, and a part of the fuel vapor evaporated in the fuel tank would leak from the canister if such undesired phenomenon takes place repeatedly.